The present invention relates to a new process for the preparation of creep-free prepregs based on phenolic resins, as well as resultant prepregs and finished molded products produced therefrom.
Prepregs are mixed materials which are prepared from thermosetting resins and reinforcing agents and, if desired, fillers. The reinforcing agents employed are in the form of fibers such as cellulose fibers, or glass fibers, in particular glass fiber rovings; nonwovens made, for example, of high molecular weight polyester, of polyvinyl chloride, glass fiber matting; or of woven material made, for example, of aromatic polyamide, of glass or of asbestos. The advantage of these prepreg materials is that they can be directly compression molded without the molder having the need beforehand to prepare a mixture of resin, catalysts, fillers and pigments. Prepregs can be classified into two categories: on the one hand, prepregs which are known as "creep-free" or "non-flowable" and, on the other hand, prepregs which are known as "creeping" or "flowable". "Creep-free" prepregs generally consist solely of glass fiber cloth reinforcements impregnated with a suitable resin taken to an appropriate stage of partial polymerization, a stage known as "B". At this stage, resins are, in practice, no longer tacky, but are still soluble and meltable. When placed in a heated press, such a mixed material allows the resin to move between the fibers and results in a uniform distribution of the glass and the resin. After the mold has been closed, the resin is set completely. Creep-free prepregs are manufactured from epoxy resins, from phenol formaldehyde resins and also from polyester resins based on diallyl phthalate.
The processes used heretofore were based on the use of solvent solutions of resins. Thus, creep-free prepregs manufactured from phenolic resins are produced from phenolic resins dissolved in a solvent, an alcohol in particular; the glass fiber cloth is impregnated with the resin in the form of solution, and this permits better dispersion of the resin in the glass cloth. The solvent is then evaporated off in an oven.
After impregnation, the mixed material is placed in a heated compression mold. Processes of this kind yield finished products which may have the disadvantage of disintegrating in use, and this greatly restricts their fields of application; they are used, in particular, principally for the manufacture of integrated circuits or of material of construction in the aeronautical field.
The major difficulty which needs to be overcome in order to manufacture these creep-free prepregs from a resin which does not contain a solvent is to have available a resin which is sufficiently fluid at 50.degree. C. so as to be capable of being properly wetting the glass cloth and which must therefore show practically no change in viscosity during the first hours at ambient temperature.
In addition, once the first 24 hours have elapsed, it is essential that this viscosity changes rapidly with time at ambient temperature, and also at a higher temperature, so that a creep-free prepreg of a suitable pegosity can be obtained. ("Pegosity" means that the viscosity value of the prepreg is such that the product sticks lightly to fingers).
It may be seen, therefore, that there is a whole series of conflicting problems to be solved, it being necessary for the solution to these problems to lead to the production of phenolic resins having a matched viscosity, but also a viscosity which is capable of changing with time in a particular manner.
A need is thus perceived to develop phenolic resins containing no solvent and being suitable for the manufacture of creep-free prepregs.
Referring now to the manufacture of creeping or flowable prepregs, also known as "prepreg mats", it is known to add a thickener chosen from alkaline-earth metal oxides such as magnesium oxide. The addition of a compound of this kind to the polyester resin makes it possible to obtain not only a resin having a desired viscosity, but also a sufficient stability which enables the resin not to thicken too quickly, with the risk of producing mediocre impregnation of the chopped glass fibers used in the creeping prepreg mat technique.
It might be reasonable to expect that a mere adaptation of the alkaline-earth metal oxides employed for polyester resins in the creeping prepreg technique to the phenolic resins employed in the creep-free prepreg technique would have made it possible to obtain phenolic resin dough compositions suitable for the manufacture of creep-free phenolic prepregs. Unfortunately, this is not the case at all. It was found, in fact, that the addition of an alkaline-earth metal oxide to phenolic resins results in resin compositions which are not homogeneous, but rather exhibiting, in particular, a surface crust. In addition, the use of an alkaline-earth metal oxide results in phenolic resins whose viscosity changes too quickly with time and which consequently can no longer be used.